runtime (gc): scan callee-saved registers while marking stack

src/internal/task/task_coroutine.go

@@ -95,3 +95,9 @@ func fake() {
 	go func() {}()
 	Pause()
 }
+
+// OnSystemStack returns whether the caller is running on the system stack.
+func OnSystemStack() bool {
+	// This scheduler does not do any stack switching.
+	return true
+}

src/internal/task/task_none.go

@@ -27,3 +27,9 @@ type state struct{}
 func (t *Task) Resume() {
 	runtimePanic("scheduler is disabled")
 }
+
+// OnSystemStack returns whether the caller is running on the system stack.
+func OnSystemStack() bool {
+	// This scheduler does not do any stack switching.
+	return true
+}

src/internal/task/task_stack.go

@@ -72,3 +72,9 @@ func start(fn uintptr, args unsafe.Pointer) {
 	t.state.initialize(fn, args)
 	runqueuePushBack(t)
 }
+
+// OnSystemStack returns whether the caller is running on the system stack.
+func OnSystemStack() bool {
+	// If there is not an active goroutine, then this must be running on the system stack.
+	return Current() == nil
+}